1. Field of the Invention
The present invention generally relates to a wireless communication system. More particularly, the present invention relates to a method and apparatus for controlling transmission and reception of dedicated pilots mapped to part of resources allocated to a data channel in a system that adaptively controls a modulation scheme and an error correction code rate.
2. Description of the Related Art
A wireless communication system has a variable-channel environment due to fading caused by the position of a Mobile Station (MS) (or user) or obstacles. A Base Station (BS) scheduler efficiently allocates resources to MSs according to their channel statuses and priority levels at every scheduling point in time, thereby improving system performance.
FIG. 1 illustrates a downlink transmission in a multi-cell wireless communication system. Referring to FIG. 1, reference numerals 110, 120 and 130 denote BSs that cover cells A, B and C, respectively. Reference numerals 111 and 100 denote MSs within cell A, reference numerals 121 and 122 denote MSs within cell B, and reference numeral 131 denotes an MS within cell C.
A measure of the channel status of each MS is Signal-to-Interference and Noise Ratio (SINR). SINR will be described with regard to the MS 131 within cell C. The reception power of a signal from the BS 130 at the MS 131 depends on fading caused by signal attenuation related to the distance between the BS 130 and the MS 131 and by an obstacle 132. A signal sent from the BS 130 in cell C is divided into a line of sight path signal component 133 and multipath signal components 134 and 135, each of which arrive at the MS 131. The multipath signal components 134 and 135 cause fading, thus varying the channel status instantaneously.
The channel status can be measured by interference and noise. This will be described with regard to the MS 112 at the boundary of cell A. The MS 112 receives a signal 123 from the BS 120 in cell B and a signal 136 from the BS 130 in cell C as well as a signal 113 from the BS 110 in cell A. The sum of the signal 113 or 136 and noise signal received from a neighbor cell is an interference and noise component. Since the MS 112 is located at the cell boundary, the ratio of interference from the neighbor cells is high and thus the MS 112 is in a poor channel status with a low SINR under the same fading environment.
Each BS is provided with a scheduler (not shown), for reliable data transmission. The scheduler selects an appropriate Modulation and Coding Scheme (MCS) according to the channel status. The MCS is given in the form of an MCS level. The scheduler selects a low MCS level specifying a low-order modulation and/or a low coding rate such as Quadrature Phase Shift Keying (QPSK) for the MS 112 with a low SINR at the cell boundary, to thereby decrease data rate and ensure reliable transmission. A low coding rate like ⅙ or 1/12 provides almost the same performance as repeated coding that repeats data bits, using more resources for transmission of the same amount of information. In this case, a receiver of the MS 112 increases SINR by combining repeated signal components.
A data channel delivers a data signal along with a control channel carrying control information and a pilot channel for estimating a channel impulse response for use in channel compensation of the data channel and the control channel at the receiver. In general, the data channel, the control channel, and the pilot channel occupy different transmission resources. The transmission resources are time resources and/or frequency resources. In Orthogonal Frequency Division Multiplexing (OFDM), the transmission resources are time-frequency resources.
FIG. 2 illustrates mapping between a data channel, a control channel, and a pilot channel to time-frequency resources in a typical OFDM system. The pilot channel applies commonly to both the data channel and the control channel and thus it is called a common pilot channel. The horizontal axis represents a time domain and the vertical axis represents a frequency domain. The time domain covers a subframe 210 and the frequency domain covers a total frequency band. A basic transmission unit in frequency is a subband 220 including one subcarrier or a set of subcarriers. One subframe carries a plurality of OFDM symbols.
In the illustrated case of FIG. 2, a common pilot channel 230 is allocated to odd-numbered subcarriers/subbands, and a control channel 240 is allocated to even-numbered subcarriers/subbands in a first OFDM symbol. The remaining resources are allocated to data channels for users, including a data channel 250 for user A.
As described before, compared to data or control information of which the received SINR can be increased by the use of a low MCS level, the pilot channel sends a predetermined amount of pilot symbols and thus there is no way to increase received pilot SINR. For a user in a poor channel status, despite an increased received SINR for data by use of a low MCS level, the degradation of channel estimation performance resulting from low pilot SINR decreases system reception performance.